Messaging Systems are known. Such systems routinely use a simulcast transmission mode for message delivery. This simulcast mode denotes a situation where all transmitters within a geographic area are active at the same time on the same radio frequency. To facilitate, indeed enable, successful communications in a simulcast mode, extreme care must be taken to insure that modulation on radio signals that originate from different transmitters and arrive at a location where the signal strength of each signal is equal are phase coherent or in phase. Equal is taken to mean within or approximating the capture ratio of a typical messaging unit or within about 6 dB for many messaging receivers when using frequency modulation.
Simulcast operation has proven to be very effective at reaching messaging receivers that are located at some unknown location within a large geographic area. However messaging systems that rely entirely on simulcast operation may be unduly capacity limited since the radio frequency used by the system can not be used at the same time for any purpose other than delivery or transmission of a single message anywhere in the system. Practitioners have realized that knowledge of a messaging unit location would allow a portion of the system, such as one transmitter, to transmit the message to the intended messaging unit. Concurrently other portions of the system, such as other transmitters may be used for other noninterfering duties, such as delivery of other messages to other messaging units, thus better utilizing system capacity.
In recognition of this some systems use digital identification (ID) signals that are unique to each transmitter providing coverage within a region. These digital identification signals or digital color codes are simulcast from each transmitter and a messaging unit with an address matching a simulcast transmitted message reports back to the system the digital ID it has received. The system thereafter attempts further or additional contemporaneous message delivery to this unit utilizing only the transmitter with that digital ID. This latter mode of operation is often designated directed or directed delivery or non-simulcast operation. While the use of digital color codes can provide a significant degree of directed message delivery and thus significant reuse of the system frequency by other transmitters within the system, certain drawbacks or limitations remain.
For example, if two signals are received each having approximately equal magnitude, the digital color codes will interfere with each other and neither may be accurately recovered. In this instance the messaging unit may not be able to determine which transmitter(s) are providing coverage. In any event, when a single digital ID is recovered it will represent the transmitter providing the best or strongest signal at the location of the messaging unit. Weaker signals with their respective digital IDs will be lost even when the weaker signal nevertheless has a completely adequate signal level. In this instance a potential opportunity or contemporaneous directed message delivery to two messaging units may be lost with a resultant reduction in system utilization. Clearly a need exists for methods and apparatus for selecting a suitable transmitter for directed message delivery in present day messaging systems.